I can still remember that it is since the 1990’s, we began to use cell phones, the number

I can still remember that it is since the 1990’s, we began to use cell phones, the number of mobile users has greatly increased．
A. while; when
B. when; what
C. before; that
D. when; that

A. while; when B. when; whatC. before; that (答案→)D. when; that解析：第一空考查定语从句，先行词是1990‘s，定语从句中缺少地点状语，用when引导定语从句，第二空是强调句，强调的是句意是：since the 1990’s，句意是：我还能记住，就是自从20世纪90年代，当时我们开始用手机了，手机用户的数量大大增加。选D.

感谢回答，我学习了